1) Field of Invention
This invention relates generally to apparatus and methods for the pumping of gases and liquids and more specifically to the field of linear pumps and compressors.
2) Description of Related Art
Over the years, efforts have been undertaken for pump and compressor designs to yield desired ideal characteristics of operation such as operation free of oils of other external lubricants, commonly known as xe2x80x9coil-free operationxe2x80x9d, variable pumping capacity, few moving parts, compatibility with a wide range of toxic or chemically reactive gases, manufacturing simplicity, size, low cost, energy efficiency, and long life. The term xe2x80x9cpumpxe2x80x9d is used herein consistent with its use by those skilled in the art to refer to both compressors and liquid pumps. The term xe2x80x9ccompressorxe2x80x9d is typically used to designate machines that compress and discharge gases such as air or refrigerants. xe2x80x9cLiquid pumpsxe2x80x9d are similar structures that typically compress the flow of a liquid. Pumps and compressors with such desired ideal characteristics have been sought for use in applications including the general compression of gases such as air, hydrocarbons, process gases, high-purity gases, hazardous and corrosive gases, as well as the compression of phase-change refrigerants for refrigeration, air conditioning and heat pumps, and other specialty vapor-compression heat transfer applications.
Prior pump design efforts have provided a diversity of pump designs that can be roughly defined in two classes of operation: positive displacement and kinetic compressors. Positive displacement compressors have been devised in two categories: (1) rotary compressors such as screws, scrolls, and rotary vanes; and (2) reciprocating compressors operating with crank-driven pistons, free-pistons, and diaphragms. Examples of kinetic compressors that have been provided are centrifugal and acoustic compressors. The operating principles of each of these compressors requires the designer to compromise or sacrifice many of the above-mentioned desired ideal characteristics in order to promote a specific characteristic in a particular design. Of particular present interest are efforts relating to free-piston compressors, diaphragm compressors and acoustic compressors.
Free-piston pumps and compressors have been designed with the hope of achieving conceptual simplicity by using a linear motor to move a reciprocating piston back and forth in its cylinder, thus eliminating crankshafts, connecting rods and bearings. However, in practice, the desired conceptual simplicity of such free-piston compressors has not been realized as other complex subsystems have been required for the operation of such free-piston compressors. For example, free-piston compressors have attempted to utilize variable capacity since the piston has no fixed displacement. With the intent of improving efficiency and capacity, such free-piston pumps have sought to operate at a resonance frequency that is defined by the piston mass and the spring stiffness of the gas-filled cylinder. However, such free-piston compressors, as with all piston compressors, require the piston to be moved very close to the head to minimize the clearance volume in the interest of volumetric efficiency. This requirement has resulted in such free-piston compressor designs experiencing undesired damage or diminished operation if the piston strikes the head during operation or during any transients that might occur. Thus, to attempt to achieve the desired characteristics in these free-piston compressor designs, elaborate and complicated controls have been required to keep the piston from striking the head during operation or during any transients that might occur. However, such controls have not satisfactorily performed under varying operational conditions.
Further, such free-piston compressors have sought to achieve oil-free operation by allowing the piston to float on a gas bearing. Unfortunately, the gas bearing has required very small clearances between the piston and cylinder, and thus high-precision machining has been required which is difficult and costly. The gas bearing also requires a network of small gas feed drillings that have a low tolerance for the moisture and particulate contamination often found in operation of such pumps and compressors. Under use conditions, such moisture and particulate contamination have caused obstructions in the small gas feed drillings that have resulted in failure or inferior performance of the gas bearings. Due to these complex subsystems that are required for operation and other reasons known in the art, these free-piston compressors have not realized certain of the desired ideal characteristics and have lacked the desired conceptual simplicity for a variety of commercial applications.
Further, attempts have been made to operate free-piston compressors at their resonance. The elements of the mass-spring resonance of certain of such free-piston compressors operated at their resonance are the compressed gas as the spring and the free piston as the mass. To take advantage of this mechanical resonance, free piston compressors must be able to accommodate the instabilities related to varying flow rates and varying compression ratios. Variations in both compression ratios and flow rates cause large variations in the spring constant of the gas. Also, low compression ratios provide little restoring force to the piston, thus causing the resonant frequency to drop below the operating frequencies needed for a given flow rate. Electromechanical and/or fluidic controls have been required in such free-piston compressors to compensate for these instabilities, thus adding complexity to the pump or compressor. Further, changing operating conditions have created an additional instability in these free-piston compressors. In operation, as the compression ratio changes, the average force exerted on the piston by the gas spring changes, thus causing the mean position of the oscillating piston to undesirably creep. This instability has also necessitated the use of various electromechanical and/or fluidic controls to stabilize the mean piston position.
In addition to free-piston compressors, diaphragm pumps and compressors have also been provided using a moving diaphragm to provide fluid compression. Attempts have been made to use such diaphragm compressors for oil-free operation by actuating such diaphragm pumps by a motor. Unfortunately, to provide the displacement needed for adequate flow rates, diaphragm compressors have typically required a non-metallic, elastic member, such as rubber, to be attached to the diaphragm. These flexible members of rubber, or other organic compounds, have been susceptible, in prior designs, to cracking, weakening, breakage or other failures of the elastic member under high pressure conditions that are necessary for the high compression ratios needed for many consumer, commercial, and industrial applications. Such susceptibility of the elastic rubber members to cracking, weakening, breakage or other failures under high pressures have reduced the reliability and life of these elastic rubber diaphragm members. Further, such elastic rubber members have not been compatible with certain fluids, such as fuels, oils, lubricants, coolants, solvents, and various chemicals, due to susceptibility of the diaphragm to cracking, weakening, degradation or failure when exposed to the fluid during operation. Certain rubber diaphragms have been used that were permeable to certain gases resulting in a flow of gas through the diaphragm and a pressure build up on the backside of the diaphragm. Also, such permeable rubber diaphragms have resulted in the contamination of the gas with rubber odors that are problematic in applications where individuals are exposed to the gas and may be allergic to the rubber odor absorbed by the gas. As such, these efforts to provide diaphragm compressors have also failed to provide the simplicity of a diaphragm design with desired characteristics in view of the required compromise in compression ratio, reliability, and application flexibility.
Certain pumps have also used valves and ports to produce flow in the pump in addition to the pressure lift to produce useful work. In typical compressors, large valves are used to provide checking action with minimized pressure loss. Such valves are typically large and relatively soft and have required mechanical stops to limit the valve""s motion. One attempt to describe a pump using non-elastomeric, flat disk springs and with valves with valve stops is described in U.S. Pat. No. 3,572,980 to Hollyday. The ""980 Patent describes a solenoid operated pump with a piston-cylinder arrangement wherein the piston is held by a flat disc spring functioning as a mechanical biasing for the piston and as a seal for the cylinder assembly. The Hollyday patent explains that a xe2x80x9cresonant operating condition is accomplished by matching the spring rate of the disc to the mass of the moving parts such that the natural frequency of the spring-mass assembly equals the driving frequency or twice the driving frequency of the energy source.xe2x80x9d
The third type of pump or compressor, the acoustic compressor, has been provided to utilize resonant operation. In such resonant operation, generally, the excitation of an empty cavity""s resonant acoustic mode creates pressure oscillations within the gas-filled cavity. These pressure oscillations have been typically converted into compression and flow by a set of reed valves that are attached to the cavity. The gas oscillates back and forth in the cavity alternately compressing and rarifying the gas. Much like a piston the displacement of this gas can be changed by varying the power input, thus resulting in variable pumping capacity. The use of resonance in resonance compressors results in high pressures and the absence of frictional moving parts to facilitate oil-free operation. However, these compressors that use acoustics as the means for providing resonance have provided disadvantages such as the large size of the cavity required to keep the operating frequencies within the range of practical compressor valves and the noise inherent in high intensity sound waves. As such, acoustic compressors tend to be physically large and noisy for a given pumping capacity, when compared to other types of compressors, which are both characteristics that can be negatives in certain commercial applications.
In summary, free-piston, diaphragm, and acoustic compressors have attempted to capture or utilize certain concepts that have the potential to provide certain of the ideal compressor characteristics described above such as variable capacity, oil-free operation, and simplicity of design. However, the current compressor designs that have sought to employ these concepts have produced many unwanted and commercially impractical disadvantages such as low compression ratios, reduced reliability, over-sized units, excessive noise, lack of fluid compatibility, need for complicated controls and high cost. Consequently, there exists a need for a pump and compressor technology that provides these ideal characteristics in an innovative manner without the historical disadvantages. As such, there also exists a need for a pump technology that can operate with the desired characteristics of oil-free operation, variable pumping capacity, few moving parts, compatibility with a wide range of toxic or chemically reactive gases, manufacturing simplicity, size, low cost, energy efficiency, and long life.
To overcome these needs and the limitations of previous efforts, the present invention is provided as a linear resonance pump for compressing fluids and includes a pump head comprising a rigid compression chamber including a wall having a geometry that defines a partial enclosure with an opening and a flexible diaphragm attached to an outer perimeter of the opening of the wall. The pump of the present invention uniquely integrates the concept of resonance with the structural simplicity of a diaphragm compressor to provide a new linear resonance pump having a wide range of improved characteristics. The pump provides fluid compression within the rigid compression chamber when the flexible diaphragm is mechanically oscillated back and forth by a motor. The pump includes tuned ports and valves that allow low-pressure fluid to enter and high-pressure fluid to exit the compression chamber in response to the cyclic compressions. The linear resonance pump also includes a motor that includes a moving portion operably connected with the diaphragm for oscillating the diaphragm at a drive frequency. The pump is desirably operated below a mechanical resonance whose frequency is determined by the moving mechanical mass of the diaphragm, a moving portion of the motor such as a piston operably connected with the diaphragm and the combined spring stiffness of the working fluid, the diaphragm, and other mechanical springs such as leaf springs connected with the moving portion.
The linear resonance pump of the present invention can be utilized in a variety of applications including the general compression of gases such as air, hydrocarbons, process gases, high-purity gases, hazardous and corrosive gases, with the compression of phase-change refrigerants for refrigeration, air conditioning and heat pumps with liquids, and other specialty vapor-compression heat transfer applications. The pump can also be utilized with liquids. The linear resonance pump can also provide variable capacity.
More specifically, one embodiment of the pump according to the present invention includes a pump head comprising a compression chamber having a wall geometry that defines a partial enclosure with an opening and a flexible diaphragm rigidly connected at an outer perimeter of the opening of the wall. The diaphragm includes a flexible portion that is free to move with respect to the outer perimeter between a plurality of first positions and a plurality of second positions, the first and second positions defining first and second volumes of the compression chamber. The pump head also includes a tuned suction port and a tuned discharge port connected in communication with the compression chamber for flowing fluid into the compression chamber through the suction port and for flowing fluid out of the compression chamber through the discharge port.
The pump also includes a fluid spring comprising the fluid that is introduced into the compression chamber being subject to varying pressure and flow conditions and a mechanical spring that comprises the diaphragm and, optionally leaf springs connected with the moving portion. In this embodiment the motor is in the form of a stator and an armature with the armature cyclable between the first positions and the second positions at a drive frequency. As the armature and diaphragm cycle into the first position the flexible portion of the diaphragm flexes to generally conform in shape to the curved section of the wall of the compression chamber for minimizing clearance volume in the compression chamber. The motor of this embodiment is a variable reluctance motor, but in other embodiments alternative motors could be used, such as motors having a piezoelectric element or a voice coil linear motor.
In operation of the pump, a mass-spring mechanical resonance frequency is determined by the combined moving masses of the moving portion and the diaphragm and by the mechanical spring and the gas spring. In the preferred embodiment, the motor is operable at a drive frequency that is less than the mechanical resonance frequency. In alternative embodiments, the motor""s drive frequency can be equal to the mechanical resonance frequency.
To facilitate the resonance operation, the pump head is desirably provided with the tuned suction port and discharge port mentioned above. The ports each have a geometry comprising a diameter, length and cross-sectional shape and the ports are each tuned by selecting the geometry of the port to achieve optimal flow resistance and timing characteristics so as to coordinate the filling and discharge of the fluid flow through the suction port and discharge port respectively in coordination with the pressure cycle in the compression chamber to provide a net flow in one direction of the fluid within the pump.
Resonant operation can be further facilitated by a valve that operatively connected to each port. For example, in this first embodiment, a discharge valve is operatively connected to the discharge port and a suction valve is operatively connected to the suction port. Each valve has a predetermined stiffness and a valve duty cycle wherein the valve prevents flow through the port in a closed position and allows flow through the port in an open position. The valves are tuned by selecting the valve stiffness and geometry, including size, such that the timing of the duty cycle of the valve is coordinated with the timing of the filling and discharge of the fluid flow through the ports and the pressure cycle in the compression chamber to provide a net flow in one direction of the fluid within the pump. The valves are adapted to each be maintained in the open position by fluid pressure differential across the valve during flow and without needing any mechanical stops. The valves operate through each of a plurality of duty cycles in a continuous motion. Tuning the valves and ports facilitates the operation of the pump at high frequencies of 100 cycles per second or greater to produce desired fluid compression. The ports can be provided as a single port, or alternatively, as a plurality of ports. The valves can be provided as a single valve for embodiments with a single port, or alternatively, with a plurality of valves corresponding to a plurality of ports. Properly tuned ports can facilitate compression and flow of the pump without valves. The addition of valves provides further enhancement of the pump""s performance.
To still further facilitate the operation of the pump at resonance and at high frequencies with high compression ratios, the pump can be provided with a hole from the compression chamber to the exterior of the compression chamber, or alternatively a plurality of holes. The hole is provided in the diaphragm, or alternatively in other parts of the pump head or pump. This hole or holes are tuned by selecting the geometry of the hole, including the size in diameter and length, to communicate a sufficient quantity of fluid through the hole for equalizing pressure on a first and second face of the diaphragm. Maintaining the equilibrium of pressure on the first and second faces of the diaphragm prevents undue stress on the diaphragm and further prevents undesirable creeping of the diaphragm""s equilibrium position, which can lead to reduced motor performance.
In a still further aspect of the pump the pump can include a single or, alternatively a plurality of leaf springs connected with the moving portion of the motor as one of the mechanical springs for providing restoring force and displacement of the moving portion such as the armature during cycling of the moving portion armature to reduce pressure on the diaphragm.
In this first embodiment of the pump, the diaphragm is made from a metal material of steel. A metal backpressure chamber can be provided in communication with the second face of the diaphragm and outside the compression chamber to provide an all-metal wetted flow path for flow of certain fluids. The use of the diaphragm allows for operation of the pump free of external lubricants. This oil free operation also allows for use of the pump irrespective of gravitational orientation for uses such as in boats or jets.
In another aspect of the present invention, the pump may also be provided with control means that are operatively connected with the linear motor for varying the drive frequency of the linear motor to oscillate the diaphragm below the mechanical resonance frequency. In alternative embodiments the control means can be used to operate the pump on the mechanical resonance frequency. The control means can be provided in alternative embodiments as a closed loop controller or an open loop controller as described below.
In still another aspect of the invention, the pump can be provided as a high frequency pump for compressing gases with tuned ports and valves as described above and which can operate at or below the mechanical resonance frequency.
In another aspect of the invention, a method for compressing a fluid using the pump is provided as follows. A similar pump as that described in the first embodiment is provided. Having provided this pump, a fluid is introduced into the compression chamber at a first pressure. This fluid acts as a fluid spring under varying pressure conditions. The mass-spring mechanical resonance frequency is determined by the combined moving masses of the moving portion of the motor and the diaphragm and by the mechanical spring including the diaphragm and leaf spring and the gas spring. The motor is operated at a drive frequency that is near and less than the corresponding mechanical resonance to cycle the moving portion and diaphragm between the first and second positions. The fluid is compressed to a desired pressure and evacuated from the compression chamber at a second pressure.
The method can further include providing the diaphragm with the hole as described, the hole being sized in diameter and length to communicate a sufficient quantity of fluid through the hole for equalizing pressure on the first and second faces; and further comprising after the oscillating step, equalizing pressure on the first and second faces of the diaphragm during said oscillation by flowing fluid through the hole. Still alternatively, the method of compressing a fluid can further comprise the step of tuning a ports such as a suction port and discharge port by selecting the sizing of each port""s geometry including the diameter, length and cross-sectional shape to coordinate the timing of the filling and discharge of the fluid flow through the ports and the pressure cycle in the compression chamber to provide a net flow in one direction of the fluid through the port. Likewise the method can include providing a tuned valve for each of the ports. Each of the valves is operatively connected to a port and has a predetermined stiffness and a valve duty cycle. The valve prevents flow through the port in a closed position and allows flow through the port in an open position. Tuning the valve comprising selecting the valve stiffness and geometry to provide a duty cycle with a timing that is coordinated with the timing of the filling and discharge of the fluid flow through the ports and the pressure cycle in the compression chamber to provide a net flow in one direction of the fluid within the pump.
The method of compressing a fluid can include in the compressing step compressing the fluid in a series of cycles at a high frequency of 100 cycles per second or greater. Further, the method can further comprise in the operating step, varying the drive frequency of the linear motor in accordance with the mechanical resonance frequency. Still further, the operating step can include varying the drive frequency by a closed loop controller or open loop controllers as described below. In these and other embodiments, the resonant operation of the linear resonance pump of the present invention provides advantages including high frequency operation, small diaphragm displacements, high compression ratios for gases, and small size. The linear resonance pump further enables the provision of a simple gas compressor with an all metal diaphragm that provides high compression ratios and also includes an all metal wetted flow path that promotes compatibility with a wide range of toxic, high-purity, reactive, or environmentally hazardous fluids. It is a still further benefit of the present invention that the linear resonance pump eliminates any frictional moving parts, thus providing oil-free operation and the freedom to operate the compressor in any physical or gravitational orientation. The linear resonance pump according to the present invention also provides high frequency resonant operation in a relatively small sized unit, and in certain embodiments can provide a resonant positive-displacement compressor with high stability under low pressure high-flow conditions. A still further benefit is that the linear resonance pump can provide a compressor with a soft start characteristic that prevents electrical current spikes upon start up.
These and other objects and advantages of the invention will become apparent from the accompanying drawings, wherein like reference numerals refer to like parts throughout.